Research on light emitting devices using an EL device as spontaneous light emitting element has been actively done. Such light emitting devices are also referred to as organic EL displays or organic light emitting diodes. Because of its features of high response speed, low voltage, and low-power-consumption drive suitable for displaying moving images, the light emitting devices attract considerable attention as next-generation displays including new-generation cellular phones and personal digital assistants (PDA).
EL devices using organic compound layers as light emitting layers have a structure in which a multilayer film is deposited on a substrate, the multilayer film including at least one layer made of an EL material (hereinafter, referred to as an EL layer) interposed between a pair of a anode and a cathode one of which has translucency and emits electro-luminescence from the EL layer by adding an electrical field to the anode and the cathode. The light emission from EL devices includes light emission (fluorescence) at electron transition from an excited singlet state to a ground state and light emission (phosphorescence) at electron transition from an excited triplet state to a ground state.
In this specification, all layers provided between a cathode and an anode are collectively referred to as EL layers.
The EL layer has a layered structure typified by “a hole transport layer/a light emitting layer/an electron transport layer.” EL materials for the EL layer are roughly classified into a low molecular (monomeric) material and a high molecular (polymeric) material.
Organic EL elements have a layered structure including an organic material between a pair of opposing electrodes and, in order to extract light generated from an EL layer, at least one of the anode and the cathode must be made of a translucent conductive material such as indium tin oxide (hereinafter, referred to as ITO). A typical organic EL element has a structure in which ITO deposited by sputtering is placed as anode on a substrate, on which an EL layer and a cathode made of metal such as aluminum are layered. With the organic EL element with such a structure, light is extracted from the anode formed on the substrate. When a display having the organic EL elements with such a structure as pixel is displayed by active matrix driving, TFTs on the substrate block off the light that has passed through the anode, significantly reducing light-emission efficiency. Accordingly, organic EL elements have been researched recently which are constructed such that a metal electrode having no translucency, on which an EL layer and a translucent conductive material are stacked. With the organic EL devices having such a structure, TFTs on the substrate do not block off light even if a display is displayed by active matrix driving.